Question

A convex lens of focal length 25 cm and a concave lens of focal length 10 cm are placed in close contact with each other. Calculate the lens power of this combination.

Answer 1

$Given,\\Focal \ length(Convex.) = 25 cm\\Focal \ length(Concave) = -10 cm\\\\ We \ know,\\\boxed{\boxed{P = \frac{1}{f}}}\\\\Now,P_1 = \frac{1}{f} = \frac{1}{25} \ cm = \frac{1}{25}\times100 \ m = 4\ D\\\\~~~~~~~P_2 =\frac{1}{f}=\frac{1}{-10} \ cm = \frac{1}{-10}\times 100 \ m= -10\ D\\\\Now,\\P =P_1 + P_2\\~~~=4+(-10) \ D\\~~~=-6 \ D\\\\\text{Power of the lens combination}=\boxed{\boxed{\bold{-6 \D}}}$

Answer 2

Given,
Focal length of convex = 25 cm.
Focal length of concave= - 10 cm.

As power formula is P= 1/ f

Now , power 1 (p1) = 1/f
= 1/ 25 cm
= 1/ 25 × 100m = 4D
Power 2(p2) = 1/ f
= 1/ -10 cm
= 1/ -10 × 100m = - 10 D
Now P = p1 p2
4(-10) D
= -6.

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